Video display device that stretches a video signal and a signal of the light source and television receiving device

ABSTRACT

A video display device increases a feeling of brightness and expresses a video with high contrast by detecting a part of a video signal that emits light, enhancing the display luminance of the light-emitting part, and displaying the part. A light emission detection portion ( 1 ) uses a prescribed feature quantity related to the brightness of an input video signal, predetermines the light emission quantity for the video signal on the basis of the relationship with the feature quantity, and detects the light emission quantity for each input video signal frame from the feature quantity. A backlight luminance stretch portion ( 3 ) stretches the light source luminance of the backlight according to the light emission quantity detected. A video signal luminance stretch portion ( 6 ) stretches the video signal according to the light emission quantity or the luminance distribution of the input video signal.

TECHNICAL FIELD

The present invention relates to a video display device and a televisionreceiver, and more particularly to a video display device and atelevision receiver provided with an enhancement function for improvingimage quality of a display video.

BACKGROUND OF THE INVENTION

An enhancement function for improving image quality of a display videoin a video display device has been known. With the enhancement function,stretch processing for expanding distribution of intensity of an inputvideo signal is performed. For example, in the case of executing theenhancement function, typically, a maximum value of a tone is detectedfor each frame of a video signal and a gain is applied to a part of thevideo signal with a high tone for expanding when the maximum value has alow level. Further, a minimum value of the tone of the video signal isdetected and a compression gain is applied to a part of the video signalwith a low tone for reducing when the minimum value is high. Using suchthe enhancement function allows a signal range of the video signal to bewider so that a contrast feeling of a display image is increased,resulting in improvement in image quality.

For example, Patent Literature 1 discloses a liquid crystal displaydevice for automatically adjusting contrast, along with adjustment ofluminance of a backlight, so that contrasting of an image closes to thatbefore the adjustment. The liquid crystal display device is configuredto change luminance of an image by turning on/off a light source of abacklight device by an operator so as to allow electricity to beeffectively saved, and since an enhancement function works according tothe changed luminance to adjust a display image with contrastcorresponding to the changed luminance, attain almost the same level ofcontrast in an image as that before lowering luminance even in the caseof lowering luminance of the backlight device.

PRIOR ART DOCUMENT Patent Documents

-   Patent Document 1: Japanese Laid-Open Patent Publication No. 9-80378

SUMMARY OF THE INVENTION Problem to be Solved by the Invention

In the case of enhancing display luminance, a bright and brilliant lightemitting part on a screen is specified to enhance display luminance ofthe light emitting part, thereby improving contrast feeling perceived bythe human eye, so that a high-definition display video with increasedfeeling of brightness can be provided.

Processing with a conventional enhancement function is performed that,according to a maximum value or a minimum value in pixel values of avideo signal, each of which is high has a tone expanded to be raisedwhile each of which is low has a tone compressed to be lowered. However,since a standardized video signal does not represent luminance actuallyperceived bright by the human eye, it is difficult to specify a lightemitting part only from a tone value. That is, even in the case ofevenly enhancing various videos according to a maximum value or aminimum value of pixel values, a high-quality video with high contrastis not necessarily obtained at all times.

The present invention has a benefit that, in widely variable videos, arelatively bright light emitting part is detected from distributedluminance of a video to consciously enhance the light emitting part sothat the light emitting part is further emphasized on a screen toimprove image quality, however, a conventional technique has notincluded enhancement function processing performed based on such anidea.

The present invention has been devised in light of the above-describedproblem, and an object of the present invention is to provide a videodisplay device and a television receiver for detecting a light emittingpart of a video signal to enhance display luminance of the lightemitting part for emphasized display, thereby representing a video withhigh contrast having further increased feeling of brightness so as toimprove video quality.

Means for Solving the Problem

To solve the above problems, a first technical means of the presentinvention is a video display device comprising: a display portion fordisplaying an input video signal; a light source for illuminating thedisplay portion; and a control portion for controlling the displayportion and the light source, wherein the control portion stretchesluminance of the light source based on a prescribed feature quantityassociated with lightness of the input video signal as well as detects alight-emitting part of the input video signal based on the featurequantity or other feature quantity, and stretches a video signal of thelight-emitting part to display on the display portion.

A second technical means is the video display device of the firsttechnical means, wherein the feature quantity is a luminance value ofthe input video signal, and the control portion detects, thelight-emitting part which is predetermined according to the histogram,based on a luminance histogram for each frame of the input video signal,and as to the input video signal of a prescribed range including thedetected light-emitting part, detects a predetermined light emissionquantity according to a score obtained by counting the number of pixelswith a weight given to luminance for each pixel to stretch the luminanceof the light source according to the detected light emission quantity.

A third technical means is the video display device of the secondtechnical means, wherein when an average value of the luminancehistogram is A and a standard deviation is σ, the control portionregards a pixel which is greater than or equal to: thresh=A+Nσ (N is aconstant) as the light-emitting part.

A fourth technical means is the video display device of the firsttechnical means, wherein the feature quantity is a maximum value of tonevalues of RGB for each pixel of the input video signal, and the controlportion detects a predetermined light emission quantity according to avalue obtained by averaging the maximum values of tone values of the RGBof the input video signal and stretches the luminance of the lightsource according to the detected light emission quantity.

A fifth technical means is the video display device of the secondtechnical means, wherein the control portion performs video processingfor converting and outputting an input tone of the input video signal,and the video processing includes processing for, based on the luminancehistogram for each frame of the input video signal, detecting thepredetermined light-emitting part according to the histogram, setting aprescribed characteristic conversion point in an area of the detectedlight-emitting part, applying a gain to a video signal with a lower tonethan the characteristic conversion point so that the input tone of theinput video signal at the characteristic conversion point is stretchedup to a prescribed output tone, and in the input tone which is greaterthan or equal to the characteristic conversion point, setting the outputtone for the input tone so as to connect the output tone afterapplication of gain at the characteristic conversion point and a maximumoutput tone.

A sixth technical means is the video display device of the secondtechnical means, wherein the control portion performs video processingfor converting and outputting an input tone of the input video signal,and the video processing includes processing for defining a relationshipbetween a gain applied to a video signal and the light emission quantityin advance, determining the gain according to the light emissionquantity detected from the input video signal, applying the determinedgain to the input video signal for stretching, using an input tone at apoint where an output tone after the application of gain is stretched upto a prescribed output tone as a characteristic conversion point,outputting the video signal with the output tone to which the gain isapplied in a lower tone than the characteristic conversion point, and inthe input tone which is greater than or equal to the characteristicconversion point, setting the output tone for the input tone so as toconnect the output tone after application of gain at the characteristicconversion point and a maximum output tone.

A seventh technical means is the video display device of the fifthtechnical means, wherein the video processing includes processing forreducing the output tone by applying a compression gain in a prescribedarea of a non-light-emitting part excluding the light-emitting partafter applying a prescribed gain to the input video signal to stretchthe video signal.

An eighth technical means is the video display device of the seventhtechnical means, wherein the compression gain is a value for reducingdisplay luminance which is increased by stretching of the luminance ofthe light source and stretching of the video signal by application ofthe gain in the prescribed area of the non-light-emitting part.

A ninth technical means is a television receiving device including thevideo display device of the first technical means.

Effect of the Invention

According to the video display device of the present invention, it ispossible to provide a video display device and a television receivingdevice that further increase a feeling of brightness and expresses avideo with high contrast by detecting a part of a video signal thatemits light and enhancing display luminance of the light-emitting part,and improve video quality by displaying the part in an emphasized mannerto thereby.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a diagram explaining an embodiment of a video display deviceaccording to the present invention, showing a configuration of a mainpart of the video display device.

FIG. 2 shows an example of a luminance histogram generated from aluminance signal Y of an input video signal.

FIG. 3 is a diagram explaining another example of detecting a lightemission quantity from a feature quantity.

FIG. 4 is a diagram showing a setting example of a luminance enhancementquantity according to a light emission quantity detected by a lightemission detection portion.

FIG. 5 is a diagram showing a control example of backlight luminanceaccording to a luminance enhancement quantity determined by a luminanceenhancement quantity determination portion.

FIG. 6 is a diagram explaining luminance stretching of a video signal ina video signal luminance stretch portion, which is a diagram showing asetting example of input/output characteristic of the video signal.

FIG. 7 is a diagram explaining another processing example of luminancestretching of a video signal in the video signal luminance stretchportion.

FIG. 8 is a diagram showing a setting example of input/outputcharacteristic in the case of stretching with a gain applied to an inputvideo signal.

FIG. 9 is a diagram showing an example of tone mapping generated by amapping portion.

FIG. 10 is a diagram showing another example of tone matting generatedby the mapping portion.

FIG. 11 is a diagram showing an example of a state where screenluminance is stretched.

FIG. 12 is a diagram explaining the effect of luminance stretchprocessing according to the present invention.

FIG. 13 is a flowchart explaining a first example of the light emissiondetection processing according to the present invention.

FIG. 14 is a flowchart explaining another example of the light emissiondetection processing according to the present invention.

FIG. 15 is a flowchart explaining an example of the luminance stretchprocessing of the backlight according to the present invention.

FIG. 16 is a flowchart explaining a first example of the luminancestretch processing of the input video signal according to the presentinvention.

FIG. 17 is a flowchart explaining another example of the luminancestretch processing of the input video signal according to the presentinvention.

PREFERRED EMBODIMENT OF THE INVENTION

Embodiment 1

FIG. 1 is a diagram explaining an embodiment of a video display deviceaccording to the present invention, showing a configuration of a mainpart of the video display device. The video display device is configuredto display a video by performing image processing for an input videosignal, and is applicable to a television receiving device and the like.

A video signal separated from a broadcast signal or a video signal inputfrom an external device is input to a light emission detection portion1. In the light emission detection portion 1, using a prescribed featurequantity associated with lightness of an input video signal, a lightemission quantity for the video signal is predetermined on the basis ofa relationship with the above-described feature quantity. Further, thelight emission quantity is detected from the feature quantity for eachframe of the input video signal.

For example, using luminance of a video signal as the feature quantity,a Y histogram obtained by integrating the number of pixels for each toneof a luminance signal Y is generated for each frame of the input videosignal to detect a light-emitting part from the Y histogram. Thelight-emitting part is obtained by an average value and a standarddeviation of the Y histogram, and is detected as a relative value foreach Y histogram.

Moreover, the light emission quantity is detected for each frame thereofby integrating the number of pixels with a larger weight given as theluminance becomes higher as to the feature quantity (luminance) of thelight-emitting part. The light emission quantity shows a degree of lightemission of the input video signal and serves as an index for performingluminance stretching of backlight and luminance stretching of the videosignal afterward.

In another example of detection of light emission by the light emissiondetection portion 1, a maximum tone value among tone values of videosignals of RGB constituting one pixel (referred to as Max RGB) isextracted, an average value of the tone values extracted from all pixelsin one frame (referred to as Max RGB Ave) is calculated, and this valueis used as the feature quantity. The Max RGB Ave of each pixel is usableas the feature quantity associated with lightness of a video. Further, arelationship between the above-described Max RGB Ave and the lightemission quantity showing a degree of light emission of the video signalis defined in advance. For example, an area where the Max RGB Ave ishigh to some extent is regarded as emitting light so that the lightemission quantity is defined to be high. Then, the light emissionquantity at the time is obtained from the above-described Max RGB Avefor each frame of the input video.

A luminance enhancement quantity determination portion 2 determines aluminance enhancement quantity used for performing luminance enhancementof the backlight based on the light emission quantity of the input videosignal detected by the light emission detection portion 1. Here, arelationship between the luminance enhancement quantity and the lightemission quantity is defined in advance, and the luminance enhancementquantity is determined based on the light emission quantity output fromthe light emission detection portion 1 in the luminance enhancementquantity determination portion 2. For example, an area where the lightemission quantity is high to some extent is defined so that theluminance enhancement quantity also becomes large. Thereby, in an imagewith a large light emission quantity, the luminance enhancement quantitybecomes higher.

A backlight luminance stretch portion 3 stretches backlight luminancebased on the luminance enhancement quantity determined by the luminanceenhancement quantity determination portion 2 to increase the luminanceof a light source (for example, LED) of a backlight portion 5. Theluminance of the LED of the backlight portion 5 is subjected to PWM(Pulse Width Modulation) control, and is able to be subjected to currentcontrol or a combination thereof to be controlled so as to have adesired value.

On the other hand, a video signal stretch portion 6 increases a gain ofthe input video signal to stretch the luminance of the video signal. Inthis case, it is possible to stretch the video signal by prescribedgain-up with respect to the light-emitting part obtained from theaverage value and the standard deviation of the luminance histogramdescribed above, or to determine the gain with the light emissionquantity calculated from the luminance histogram and the Max RGB Ave tostretch the video signal.

A mapping portion 7 generates tone mapping of input/outputcharacteristic (response characteristic of an output tone for an inputtone) of the video signal. In this case, when the gain determined by thevideo signal luminance stretch portion 6 is applied as it is to performtone mapping of the input/output characteristic, an area excluding thelight-emitting part of the video signal is also stretched to increasethe screen luminance. Therefore, in a non-light-emitting part on the lowtone side, tone mapping is performed with the output tone for the inputtone reduced. Thereby, in the input/output characteristic of tonemapping, an area where the video signal is stretched mainly becomes abright area with a high tone and control is performed to make the brightarea much brighter by video signal processing.

The mapping portion 7 outputs control data for controlling a displayportion 9 to a display control portion 8, and the display controlportion 8 controls display on the display portion 9 based on the data.For the display portion 9, a liquid crystal panel that displays an imageby illumination with the LED of the backlight portion 5 is used.

With the above-described configuration, since the luminance stretchquantity of the backlight portion 5 is determined based on the lightemission quantity detected by the light emission detection portion 1, itis possible to perform control to brighten the bright video with largelight emission quantity much brighter. Moreover, gain-up of the videosignal by video signal processing is performed according to alight-emitting area of the Y histogram and the detected light emissionquantity, and further, the luminance is reduced for a part regarded asnot emitting light excluding the light-emitting part through tonemapping. This makes it possible to increase the screen luminance for alight-emitting part and express a video with high contrast so that imagequality is able to be improved.

As a control example of the backlight portion 5 and the display portion9, it is possible to adopt a so-called area active control system thatdivides a video area into multiple areas (areas) to control a lightsource of the corresponding backlight portion 5 for each of the areas.

In the area active control, a video is divided into prescribed multipleareas (areas) and a maximum tone value of a video signal is extractedfor each of the divided areas to determine a lighting rate of an LED foreach area according to the extracted maximum tone value. Here, it maynot be the maximum tone value for each of the divided areas and may beother statistical value such as an average value for each of the dividedareas. Moreover, for example, for a dark area where the maximum tonevalue is low, the lighting rate is decreased to reduce the luminance ofthe backlight. Then, electricity powered to the entire backlight isincreased according to the luminance enhancement quantity in this stateto entirely increase the luminance of the backlight. Thereby, a brightvideo that emits light is made brighter and a feeling of brightness isincreased.

Moreover, in a non-light-emitting part, the luminance corresponding toluminance stretching is reduced by video signal processing, resultingthat the luminance only for a light-emitting part becomes high on ascreen, so that a high-quality video with high contrast is able to bedisplayed.

In addition, as a control example of the backlight portion 5 and thedisplay portion 9, the light emission luminance of the entire lightsource of the backlight portion 5 may be stretched according to theluminance enhancement quantity determined by the luminance enhancementquantity determination portion 2, without applying the area activecontrol system as described above. Thereby, a bright video that emitslight is made brighter and a feeling of brightness is increased.Moreover, in a non-light-emitting part, the luminance corresponding toluminance stretching is reduced by video signal processing, resultingthat the luminance for a light-emitting part becomes high on a screen,so that a high-quality video with high contrast is able to be displayed.

Note that, a control portion of the present invention is for controllingthe backlight portion 5 and the display portion 9, and corresponds tothe light emission detection portion 1, the luminance enhancementquantity determination portion 2, the backlight luminance stretchportion 3, the backlight control portion 4, the video signal luminancestretch portion 6, the mapping portion 7, and the display controlportion 8.

In the case of constituting the above-described display device as atelevision receiving device, the television receiving device is providedwith means to select a broadcast signal received by an antenna to bedemodulated, followed by decoding, for generating a video signal forreproduction, in which prescribed image processing is appropriatelyapplied to the video signal for reproduction to be input as the inputvideo signal of FIG. 1. This makes it possible to display the receivedbroadcast signal on the display portion 9. The present invention is ableto be configured as a video display device and a television receivingdevice provided with the video display device.

Hereinafter, description will be given in more detail for a processingexample of each portion of the present embodiment having theabove-described configuration.

First, description will be given in more detail for light emissiondetection processing in the light emission detection portion 1.

As described above, in the light emission detection portion 1, using aprescribed feature quantity associated with lightness of an input videosignal, a light emission quantity for the video signal is predeterminedon the basis of a relationship with the above-described featurequantity. Further, the light emission quantity is detected from thefeature quantity for each frame of the input video signal.

(Light Emission Detection Processing 1)

A first example of light emission detection processing is shown in theflowchart of Fig. 13. In the first example of light emission detectionprocessing, using luminance of a video signal as the feature quantity,in Step 13-1, a luminance histogram obtained by integrating the numberof pixels according to a luminance level is generated for each frame ofthe input video signal to detect a light-emitting part for each framefrom the histogram.

FIG. 2 shows an example of a luminance histogram generated from aluminance signal Y of an input video signal. The light emissiondetection portion 1 integrates the number of pixels for each luminancetone to generate a Y histogram for each frame of the input video signal.A horizontal axis indicates a tone value of luminance Y, and a max valuehas a tone level of 255, for example, in the case of a video signal of8-bit representation. A vertical axis indicates the number of pixelsintegrated for each tone value (frequency). When the Y histogram isgenerated, in Step 13-2, an average value (Ave) and a standard deviation(σ) are calculated from the Y histogram, which are used for calculatingtwo thresholds Th.

A second threshold Th2 is for defining a light emitting boundary. In theY histogram, in Step 13-3 processing is performed for detecting pixelswhich are greater than or equal to the threshold Th2 as being alight-emitting part.

The second threshold Th2 is provided by:Th2=Ave+Nσ  expression (1)N is given as a prescribed constant.

Additionally, a first threshold Th1 is set so as to suppress incongruityin tones of areas which are smaller than Th2 and the like, and providedby:Th1=Ave+Mσ  expression (2)M is given as a prescribed constant, and M<N.

Further, in the present example,in step 13-4, a third threshold Th3 isfurther set. The third threshold Th3 is placed between Th1 and Th2, andis provided for detecting a light emission quantity. The light emissionquantity is for defining a degree of light emission of thelight-emitting part as an index, and is predetermined on the basis of arelationship with the feature quantity. In the present example, thelight emission quantity is calculated as a score by calculation shownbelow.

The threshold Th3 may have the same value as Th2, but is provided havinga large margin for a light-emitting part whose value is greater than orequal to Th2 in order to easily perform processing. Therefore, Th3 isgiven as,Th3=Ave+Qσ(M<Q≦N)  expression (3)

In Step 13-5, the score (light emission quantity) shows a degree oflightness by counting the number of pixels of pixels with a tone valuewhich is greater than or equal to the third threshold Th3 to calculate aweighted distance from the threshold Th3, and, in Step 13-6, iscalculated, for example, by:Score=1000×Σcount[i]×(i ²−Th3²)/(Σcount[i]×Th3 ²)  expression (4)Σcount[i] is obtained by counting the number of pixels for each tonevalue i for integration. Therefore, when there are a lot of high tonepixels away from Th3 in a light-emitting part, the score becomes high.Furthermore, even when the number of pixels which are greater than orequal to Th3 is fixed, the score becomes higher when there are a lot ofhigh tone pixels.(Light Emission Detection Processing 2)

Another example of light emission detection is shown in the flowchart ofFig. 14. FIG. 3 is a diagram explaining the another example of detectinga light emission quantity from a feature quantity. In the presentexample, in Step 14-1, used as the feature quantity of an input videosignal is a value obtained by averaging maximum tone values (Max RGB)among tone values of video signals of RGB constituting one pixel in allpixels in a frame (Max RGB Average (Max RGB Ave)).

Further, as shown in FIG. 3, a relationship between the detected Max RGBAve and the light emission quantity (score) is predetermined, and inStep 14-2, the predetermined relationship of Fig. 3 is provided. In thisexample, in an area from C0 where Max RGB Ave is minimum to anintermediate point C1, the light emission quantity (score) is 0. Thatis, this area is regarded as not emitting light. Moreover, in an area C1to C2 (C1<C2), the light emission quantity is also increased accordingto an increase in Max RGB Ave. The light emission quantity becomes fixedat a maximum level from C2 to C3 (maximum value of Max RGB Ave).

In the light emission detection portion 1, in Step 14-3, the lightemission quantity (score) according to the detected Max RGB Ave isdetermined in accordance with the predetermined relationship as shown inFIG. 3.

An example of luminance stretch processing of the backlight is shown inthe flowchart of Fig. 15. FIG. 4 is a diagram showing a setting exampleof a luminance enhancement quantity according to a light emissionquantity detected by the light emission detection portion. In theluminance enhancement quantity determination portion 2, in Step 15-1, aluminance enhancement quantity used for luminance stretching of thebacklight is calculated in accordance with the light emission quantity(score) detected by the light emission detection portion 1. Theluminance enhancement quantity is set as a quantity showing the maximumluminance that is desired to be displayed, and is able to be shown witha value of screen luminance (cd/m²) and a magnification of luminancestretching and the like, for example. The maximum luminance that isdesired to be displayed is the screen luminance, for example, when avideo signal has the maximum tone (tone level of 255 in the case of8-bit representation).

In the example of FIG. 4, while the light emission quantity is at afixed level or higher (D2 to D3 (maximum value of light emissionquantity)), the luminance enhancement quantity is set to be fixed at ahigh level to increase a feeling of brightness by stretching a brillianthigh-tone video so as to have higher luminance. In this example, in apart where the score is at a fixed level or higher, possible maximumscreen luminance reached after luminance stretching is set to 1500(cd/m²), for example. Moreover, an area where the light emissionquantity is lower than D2 (D1 (D1<D2) to D2) is set so that theluminance stretch quantity becomes smaller as the light emissionquantity becomes smaller. In an area where the light emission quantityis minimum (D0 (light emission quantity=0) to D1), luminance enhancementis not performed. This is because the light emission quantity is smallso that there is less light-emitting part and little effect is giveneven when luminance enhancement is performed. The maximum screenluminance in this case is set to, for example, 450 cd/m².

(Luminance Stretch Processing of Backlight)

FIG. 5 is a diagram showing a control example of backlight luminanceaccording to a luminance enhancement quantity determined by theluminance enhancement quantity determination portion.

In the backlight luminance stretch portion 3, in Step 15-2, theluminance enhancement quantity determined according to a light emissionquantity (score) of a video signal by the luminance enhancement quantitydetermination portion 2 is used to stretch light source luminance of thebacklight portion 5. Here, the backlight control portion 4 controls thebacklight portion 5 in accordance with the luminance stretch quantitydetermined by the backlight luminance stretch portion 3.

The luminance stretching is performed, for example, in accordance withthe prescribed characteristic of FIG. 5. In FIG. 5, a horizontal axisindicates a luminance enhancement quantity determined by the luminanceenhancement quantity determination portion 2, and a vertical axisindicates a luminance level of the backlight, which is defined, forexample, by driving duty, a driving current value or the like of thebacklight.

For example, the maximum screen luminance in the case of normal lightingwithout stretching the backlight luminance is set to 450 cd/m². Here,when the light emission quantity is small and there is few area thatemits light, the luminance enhancement quantity is at a minimum leveland the light emission luminance level of the backlight is controlled ata point E1 of FIG. 5.

As the light emission quantity becomes higher from a state correspondingto the point E1, the luminance enhancement quantity also increases. Inthis case, the light emission luminance of the backlight is greatlystretched according to an increase in the luminance enhancement quantityas shown in FIG. 5. At a point E2 where the luminance enhancementquantity has a maximum value, the light emission luminance of thebacklight is stretched so that the maximum screen luminance becomes 1500cd/m², for example. Such control makes it possible to increase a feelingof brightness of a light-emitting part by stretching the light emissionluminance of the backlight according to the light emission quantitydetected from the input video signal.

(Luminance Stretch Processing of Video Signal 1)

Fig. 16 is a flowchart for a example of luminance stretch processing ofthe input video Signal. FIG. 6 is a diagram explaining luminancestretching of a video signal in the video signal luminance stretchportion, which is a diagram showing a setting example of input/outputcharacteristic of the video signal.

As described above, in the light emission detection portion 1, aluminance (Y) histogram of an input video signal is generated todetermine the second threshold Th2 for defining a light emittingboundary based on an average value and a standard deviation thereof.Pixels which are greater than or equal to the threshold Th2 in the Yhistogram are regarded as light-emitting parts. In the video signalluminance stretch portion 6, video processing for stretching the videosignal of the light-emitting part is performed.

At the time, input/output characteristic of the video signal is set asshown in FIG. 6 as an example. In FIG. 6, a horizontal axis indicates aninput tone of the luminance Y of the video signal and a vertical axisindicates an output tone according to the input tone. Moreover,input/output characteristic of an RGB signal may be defined instead ofthe luminance Y. In the case of the RGB signal, a gain shown below isapplied to each of the RGB signal to define input/output characteristic.The maximum value of the input/output tones has a tone level of 255 inthe case of a video signal of 8-bit representation, for example. In FIG.6, T1 indicates input/output characteristic after luminance stretchprocessing.

In setting of the input/output characteristic T1, first, in Step 16-1, apoint I1 of the input tone is defined. The point I1 is set at aprescribed arbitrary position which is defined in advance. Theprescribed position does not change dynamically according to the secondthreshold Th. Accordingly, when the position of the point I1 is on thelower-tone side than the second threshold Th2, the point I1 becomes thesame value as that of the second threshold Th2. The point I1 correspondsto a characteristic conversion point of the present invention.

At the time, in Step 16-2, an output tone O1 for the input I1 is set toa prescribed value in advance. For example, it is set to a position tobe 80% of a maximum value O2 of the output tone. Accordingly, in theinput/output characteristic T1, in Step 16-3, a fixed gain G1 is appliedto the input video signal for stretching so that the input tone of thepoint I1 becomes the output tone O1 in areas with the input tone from 0to I1. The gain G1 is able to be expressed as an inclination of theinput/output characteristic T1. The gain G1 is determined according tothe position of I1 in which the output tone is determined.

Further, in Step 16-4, it is set so that a maximum output value O2 thathas the same tone as the input tone is output in the case of a maximuminput tone I2, and the output tone position corresponding to the pointI1 and the output position tone position corresponding to the maximuminput value I2 are connected linearly from the input tone I1 to themaximum tone I2. In the area from I1 to I2, increasing the outputluminance gradually as the input tone becomes higher in a state whereluminance stretching is performed sufficiently at I1 enables to preventclipped whites after the luminance stretching as much as possible toexpress tones.

Thereby, the input/output characteristic T1 as shown in FIG. 6 isdefined. With stretching of the video signal at the time, the luminanceof the video signal of a light-emitting part is stretched, but anon-light-emitting part with a low tone is also stretched, so that tonemapping processing for reducing the luminance of the video signal of thenon-light-emitting part again is performed in the mapping portion 7 at asubsequent stage.

(Luminance Stretch Processing of Video Signal 2)

Fig. 17 is a flowchart for another example of luminance stretchprocessing of the input video signal. FIG. 7 is a diagram explaininganother processing example of luminance stretching of a video signal inthe video signal luminance stretch portion. In the processing example 1shown in FIG. 6, the point I1 which is to have a prescribed output tonevalue is provided according to the Y histogram of the video signal, anda gain applied to the input video signal is set accordingly.

On the other hand, in the case of the present processing example, thegain for stretching the video signal is set based on a value of thelight emission quantity (score) detected by the light emission detectionportion 1 according to the Y histogram or Max RGB Ave.

As shown in FIG. 7, the video signal luminance stretch portion 6 definesa relationship between the light emission quantity and the gain inadvance. This relationship is provided in Step 17-1. Further, an LUTwhich defines the relationship therebetween is created to determine thegain according to the light emission quantity with this LUT. Here,basically, as the light emission quantity becomes higher, the gain forstretching the video signal is increased. Moreover, it is possible toset so as not to increase the gain in a prescribed area with a smalllight emission quantity. This is because there is less light-emittingpart when the light emission quantity is small and little effect isgiven even when luminance stretching of the video signal is performed.

FIG. 8 is a diagram showing a setting example of input/outputcharacteristic in the case of stretching with a gain applied to an inputvideo signal. In Step 17-2, the video signal luminance stretch portion 6determines a gain from a light emission quantity based on therelationship shown in FIG. 7 to apply to a video signal. For example,with the relationship of FIG. 7, a gain G2 is to be determined.

In this case, as shown in FIG. 8, the above-described gain G2 which isdetermined is applied, in Step 17-3, to the input video signal in arange where an input tone is from minimum (0) to a prescribed tone I3.The gain G2 is expressed as an inclination quantity of input/outputcharacteristic T2 after the gain is applied.

The prescribed tone I3 is able to be set arbitrarily. For example, theoutput tone O3 corresponding to the input tone I3 is set to a tone to be80% of a maximum tone O4. Further, I3 is given as the input tone whenthe gain G2 is applied to the video signal and the output tone reaches80% of the maximum tone. In Step 17-4, the output tone position of I3and the output tone position of the maximum tone I4 are connectedlinearly when the input tone is between I3 and the maximum tone I4.Thereby, the input/output characteristic T2 as shown in FIG. 8 isdefined. I3 corresponds to a characteristic conversion point of thepresent invention.

With stretching of the video signal at the time, the luminance of thevideo signal of a light-emitting part is stretched, but anon-light-emitting part is also stretched, so that video processing forreducing the luminance of the video signal of the non-light-emittingpart again is performed in the mapping portion 7 at a subsequent stage.

(Mapping Processing 1)

As described above, in the backlight luminance stretch portion 3, thestretch quantity to stretch the luminance of the backlight portion 5 isdetermined in accordance with the luminance enhancement quantitydetermined according to the light emission quantity. Moreover, in thevideo signal luminance stretch portion 6, the video signal is stretchedbased on a state of distribution of the Y histogram or the detectedlight emission quantity. Accordingly, if nothing is done, the luminanceincreases in all tone areas of the input video signal, and so-calledblack floating easily occurs so that quality is decreased as well as acontrast feeling lacks.

In the mapping portion 7, the luminance of a non-light-emitting part isreduced by video signal processing. Thereby, the luminance of alight-emitting part of the input video signal is stretched so that theluminance of the non-light-emitting part is not changed to give acontrast feeling and emphasize a feeling of brightness of thelight-emitting part.

FIG. 9 is a diagram showing an example of tone mapping generated by themapping portion 7, which is a diagram showing an example of tone mappingwhen a video signal is stretched in accordance with the position of I1set to the Y histogram of the video signal by the luminance stretchprocessing 1 shown in FIG. 6. In FIG. 9, a horizontal axis indicates aninput tone of the video signal and a vertical axis indicates an outputtone. The input/output tones are able to be luminance Y of the videosignal or tones of RGB. In the case of the RGB signal, a gain shownbelow is applied to each of the RGB signal to define input/outputcharacteristic.

An area which is greater than or equal to the second threshold Th2 thatis detected by the light emission detection portion 1 is a part that isregarded as emitting light in the video. In the mapping portion 7,inStep 16-5 (17-5), a compression gain is applied to the video signalsubjected to luminance stretching in the video signal luminance stretchportion 6 excluding a light-emitting part to perform mapping ofcharacteristic subjected to gain-down.

At the time, when a fixed compression gain is uniformly applied to anarea which has a lower tone than Th2 serving as a light emittingboundary to suppress the output tone, there is sense of incongruityarising in tones. Therefore, the first threshold Th1 is set and a gainG3 is set for an area which has a lower tone than Th1 is set to performtone mapping so as to linearly connect between Th1 and Th2.

The gain G3 is for compensating for and reducing the luminancecorresponding to both of the luminance stretch quantity by the backlightluminance stretch portion 3 and the luminance stretch quantity by thevideo signal luminance stretch portion 6, and is set to a value thatmaintains the tone of the input video signal on a screen.

Here, it is set that the backlight luminance is subjected to luminancestretching by b-times. A reference of b-times is the backlight luminanceof the point E1 of FIG. 5, and shows by how many times of the luminanceat the time the luminance stretching is performed. In this case, whenreducing and compensating for the b-times backlight luminance stretchquantity by video signal processing, a required reduction amount becomes(1/b)^(γ)-times.

Moreover, it is set that the luminance stretch quantity by the gain G1in the video signal luminance stretch portion 6 is a-times. A referenceof a-times is input/output characteristic in the case of gain=1 (inputtone=output tone). In this case, the luminance reduction amount by videoprocessing of the mapping portion 7 becomes 1/a-times. Accordingly, thegain G3 which is applied to an area smaller than the first threshold Th1is set by (1/b)^(γ)×(1/a). Thereby, the screen luminance according tothe tone of the input video signal is maintained in a range of the lowertone than the first threshold Th1 among a non-light-emitting part of theinput video signal.

In the tone mapping with the second threshold Th2 or more, theinput/output characteristic stretched in the video signal luminancestretch portion 6 is used as it is. A characteristic conversion point(knee point) of the input/output characteristic in the input tone I1 setto the second threshold Th2 or more is also maintained as it is.Thereby, in an area of a light emitting color with the second thresholdTh2 or more, a light image with a feeling of brightness is obtained bystretching of the video signal and luminance stretching of thebacklight.

Further, it is set so that the output tone of the first threshold Th 1reduced by the gain G3 and the output tone of the second threshold Th2are connected with a straight line from the first threshold Th1 to thesecond threshold Th2 . By the above-described processing, tone mappingas shown in FIG. 9 is obtained. At the time, for a connecting part ofTh1 and Th2 and the characteristic conversion point of the input toneI1, a predetermined range (for example, connecting part±Δ (Δ is aprescribed value)) may be subjected to smoothing by a quadraticfunction.

(Mapping Processing 2)

FIG. 10 is a diagram showing another example of tone mapping generatedby the mapping portion 7, which is a diagram showing a tone mappingexample when a video signal is stretched in accordance with a gain setfrom a light emission quantity of the video signal by the video signalluminance stretch processing shown in FIG. 8. In FIG. 10, a horizontalaxis indicates an input tone of the video signal and a vertical axisindicates an output tone. The input/output tones are able to beluminance Y of the video signal or tones of RGB. In the case of the RGBsignal, a gain shown below is applied to each of the RGB signal todefine input/output characteristic.

In the present example as well, similarly to the first processingexample of FIG. 10, a compression gain is applied to the video signalsubjected to luminance stretching in the video signal luminance stretchportion 6 excluding a light-emitting part to apply gain-down. In thiscase, similarly to the example of FIG. 9, the first threshold Th1 is setand the gain G3 is set for an area smaller than Th1 is set to performtone mapping so as to linearly connect between Th1 and Th2.

The gain G3 is for reducing the luminance corresponding to both of theluminance stretch quantity by the backlight luminance stretch portion 3and the luminance stretch quantity by the video signal luminance stretchportion 6, and when the backlight luminance is subjected to luminancestretching by b-times and the luminance stretch quantity by the gain G2in the video signal luminance stretch portion 6 is a-times, the gain G3which is applied to the area smaller than the first threshold Th1becomes (1/b)^(γ)×(1/a). Thereby, the screen luminance according to thetone of the input video signal is maintained in an area with the lowertone than the first threshold Th1 among a non-light-emitting part of theinput video signal.

Moreover, in the tone mapping with the second threshold Th2 or more, theinput/output characteristic stretched in the video signal luminancestretch portion 6 is used as it is. Thereby, in an area of a lightemitting color with the second threshold Th2 or more, a light image witha feeling of brightness is obtained by stretching of the video signaland luminance stretching of the backlight.

Further, it is set so that the output tone of the first threshold Th1reduced by the gain G3 and the output tone of the second threshold Th2are connected with a straight line from the first threshold Th1 to thesecond threshold Th2. By the above-described processing, tone mapping asshown in FIG. 10 is obtained. A characteristic conversion point (kneepoint) of the input tone I3 set in the video signal luminance stretchportion 6 is not maintained in the case of being smaller than the secondthreshold Th2 and taken into by the line connecting the output tones ofthe first threshold Th1 and the second threshold Th2. Accordingly, a newcharacteristic conversion point is set to the output tone part of thesecond threshold Th2. At the time, for a connecting part of Th1 and Th2,a predetermined range (for example, connecting part±Δ (Δ is a prescribedvalue)) may be subjected to smoothing by a quadratic function.

FIG. 11 is a diagram showing an example of a state where screenluminance is stretched. A horizontal axis indicates a tone value of aninput video signal and a vertical axis indicates screen luminance(cd/m²) of the display portion 9.

S1, S2 and S3 correspond to a tone value of a minimum tone, a tone valueof the first threshold Th1, and a tone value of the second thresholdTh2, respectively. In the case of the input tone values from S1 to S2,tone mapping of the video signal is performed so as to reduce amount forthe screen luminance increased by the luminance stretching of thebacklight and the stretching of the video signal as described above.Therefore, a screen is displayed with a first γ curve (γ1) from S1 toS2. The first γ curve (γ1) is such standard luminance that the screenluminance becomes 450 cd/m² in the case of the maximum tone value.

In the case of a dark video with a low tone, displaying with increasedluminance causes reduction of the contrast and deterioration of thequality such as black floating, and the screen luminance is thus notincreased by suppressing the luminance only by amount of luminancestretching of the backlight and amount of luminance stretching of thevideo signal by video signal processing. Note that, the γ curve from S1to S2 does not need to conform to the above-described standard first γcurve (γ1), and is able to be set by appropriately adjusting the gainG3, as long as having a level allowing a difference from a stretch areaof a light-emitting part.

Moreover, from S2 to S3, according to tone mapping of Th1 to Th2, thescreen luminance increases being away from the first γ curve (γ1) withthe increase in the input tone, and increases up to a level of a secondγ curve (γ2) near S3 corresponding to the second threshold Th2.Thereafter, an increasing rate of the screen luminance decreases(inclination becomes gradual) and the input tone reaches maximum. Thesecond γ curve (γ2) shows the screen luminance with the γ curve when thevideo signal is stretched with the gain G1 of FIG. 6 or the gain G2 ofFIG. 8. Moreover, by decreasing the increasing rate of the screenluminance in a higher tone area than S3, deformation in the high tonearea by the luminance stretching is prevented to maintain tonerepresentation as much as possible. In this manner, it is possible toexpress a high-quality video having a feeling of brightness and acontrast feeling in the high tone area.

FIG. 12 is a diagram explaining the effect of luminance stretchprocessing according to the present invention, which is a diagramshowing an example of a state of a luminance histogram before and afterluminance stretch processing. In FIG. 12, h1 indicates a luminancehistogram obtained from an input video signal before luminance stretchprocessing is performed, h2 indicates a luminance histogram when tonemapping is performed for the luminance histogram h1 by theabove-described luminance stretching 1 and mapping processing 1, and h3indicates a luminance histogram when tone mapping is performed for theluminance histogram h1 by the above-described luminance stretching 2 andmapping processing 2.

In this example, in the luminance histogram h1 of the input videosignal, a lot of pixels exist in a low tone area smaller than the firstthreshold Th1 and significant pixels exist also in a high tone areagreater than the second threshold Th2. That is, an image is such that alight part that is regarded as emitting light exists in a relativelydark screen.

In the luminance stretching 1 and the mapping processing 1, the secondthreshold Th2 is set from a luminance histogram of an input video signaland a gain is increased for an area from the minimum tone to the pointI1 which is greater than or equal to Th2 to reduce the luminance of thelower tone area than the first threshold Th1 that is anon-light-emitting part by mapping processing. In the luminancehistogram h2 obtained by this processing, the non-light-emitting partwith low luminance is not subjected to luminance stretching so that thetone of the input video signal is maintained. Moreover, a pixel groupthat emits light on the higher tone side than a light emitting boundaryof Th2 shifts to the further higher tone side by luminance stretching.That is, only the light-emitting part is subjected to luminancestretching to increase a contrast feeling and a feeling of brightness.

In the luminance stretching 2 and the mapping processing 2, a gain isdetermined based on a light emission quantity detected from an inputvideo signal, the determined gain is applied to a low tone area forincreasing the gain, and the luminance of a lower tone area than thefirst threshold Th1 that is a non-light-emitting part is reduced by themapping processing.

In the luminance histogram h3 obtained by this processing, similarly tothe histogram h2, the non-light-emitting part with low luminance is notsubjected to luminance stretching so that the tone of the input videosignal is maintained, however, the pixel group that is regarded asemitting light on the high tone side shifts to the further higher toneside than the histogram Th2. Through the luminance stretching isperformed with the threshold (second threshold Th2) that is set based ona distribution state of the luminance histogram (Ave., σ) in theluminance stretching 1 and the mapping processing 1, the number ofpixels of the light-emitting part is weighted for integration, and basedon which, the luminance stretching is performed in the luminancestretching 2 and the mapping processing 2. Accordingly, in the case of avideo that has a lot of pixel groups in a high tone area and the like,the luminance stretch quantity becomes large, a feeling of brightness isfurther increased than the case of the histogram Th2, and a contrastfeeling is improved.

The above-described examples show an example of a state of a video whenexcellent effect is obtained, and even in any case of the processing, itbecomes possible to express a high-quality video by improving a contrastfeeling and increasing a feeling of brightness of a light part withluminance stretching of the backlight, luminance stretching and tonemapping of the video.

EXPLANATIONS OF LETTERS OR NUMERALS

1 . . . light emission detection portion, 2 . . . luminance enhancementquantity determination portion, 3 . . . backlight luminance stretchportion, 4 . . . backlight control portion, 5 . . . backlight portion, 6. . . video signal luminance stretch portion, 7 . . . mapping portion, 8. . . display control portion, and 9 . . . display portion.

The invention claimed is:
 1. A video display device comprising: adisplay portion for displaying an input video signal; a light source forilluminating the display portion; and a control portion for controllingthe display portion and the light source, wherein the control portiondetects a light-emitting part of the input video signal based on aluminance value of the input video signal or other feature quantity, andstretches a video signal of the light-emitting part to display on thedisplay portion, the control portion detects a light-emitting part whichis predetermined according to a histogram, based on a luminancehistogram for each frame of the input video signal, and as to the inputvideo signal of a prescribed range including the detected light-emittingpart, detects a predetermined light emission quantity according to ascore obtained by counting the number of pixels with a weight given toluminance for each pixel to stretch the luminance of the light sourceaccording to the detected light emission quantity, the control portionregards a pixel which is greater than or equal to:thresh=A+Nσ (N is a constant) as the light-emitting part, where anaverage value of the luminance histogram is A and a standard deviationis σ.
 2. A television receiving device including the video displaydevice as defined in claim
 1. 3. A video display device comprising: adisplay portion for displaying an input video signal; a light source forilluminating the display portion; and a control portion for controllingthe display portion and the light source, wherein the control portiondetects a light-emitting part of the input video signal based on amaximum value of tone values of RGB for each pixel of the input videosignal or other feature quantity, and stretches a video signal of thelight-emitting part to display on the display portion, and the controlportion detects a predetermined light emission quantity according to avalue obtained by averaging the maximum values of tone values of the RGBof the input video signal and stretches the luminance of the lightsource according to the detected light emission quantity.
 4. A videodisplay device comprising: a display portion for displaying an inputvideo signal; a light source for illuminating the display portion; and acontrol portion for controlling the display portion and the lightsource, wherein the control portion detects a light-emitting part whichis predetermined according to a histogram, based on a luminancehistogram for each frame of the input video signal, and as to the inputvideo signal of a prescribed range including the detected light-emittingpart, detects a predetermined light emission quantity according to ascore obtained by counting the number of pixels with a weight given toluminance for each pixel to stretch luminance of the light sourceaccording to the detected light emission quantity, the control portionperforms video processing for converting and outputting an input tone ofthe input video signal, and the video processing includes processingfor, based on the luminance histogram for each frame of the input videosignal, detecting the predetermined light-emitting part according to thehistogram, setting a prescribed characteristic conversion point in anarea of the detected light-emitting part, applying a gain to a videosignal with a lower tone than the characteristic conversion point sothat the input tone of the input video signal at the characteristicconversion point is stretched up to a prescribed output tone to displayon the display portion, and in the input tone which is greater than orequal to the characteristic conversion point, setting the output tonefor the input tone so as to connect the output tone after application ofgain at the characteristic conversion point and a maximum output tone.5. The video display device as defined in claim 4, wherein the videoprocessing includes processing for reducing the output tone by applyinga compression gain in a prescribed area of a non-light-emitting partexcluding the light-emitting part after applying a prescribed gain tothe input video signal to stretch the video signal.
 6. The video displaydevice as defined in claim 5, wherein the compression gain is a valuefor reducing display luminance which is increased by stretching of theluminance of the light source and stretching of the video signal byapplication of the gain in the prescribed area of the non-light-emittingpart.
 7. A video display device comprising: a display portion fordisplaying an input video signal; a light source for illuminating thedisplay portion; and a control portion for controlling the displayportion and the light source, wherein the control portion detects alight-emitting part which is predetermined according to a histogram,based on a luminance histogram for each frame of the input video signal,and as to the input video signal of a prescribed range including thedetected light-emitting part, detects a predetermined light emissionquantity according to a score obtained by counting the number of pixelswith a weight given to luminance for each pixel to stretch luminance ofthe light source according to the detected light emission quantity, thecontrol portion performs video processing for converting and outputtingan input tone of the input video signal, and the video processingincludes processing for defining a relationship between a gain appliedto a video signal and the light emission quantity in advance,determining the gain according to the light emission quantity detectedfrom the input video signal, applying the determined gain to the inputvideo signal for stretching, using an input tone at a point where anoutput tone after the application of gain is stretched up to aprescribed output tone as a characteristic conversion point, outputtingthe video signal with the output tone to which the gain is applied in alower tone than the characteristic conversion point, and in the inputtone which is greater than or equal to the characteristic conversionpoint, setting the output tone for the input tone so as to connect theoutput tone after application of gain at the characteristic conversionpoint and a maximum output tone.